What is the different betwen "Domain not available" vs "Domain already registered"?

[Vino]

When i tried to reserve domain names most appears "Domain already registered" and your offered alternative choice or other extensions.

But few domains appear "Not available" and no other alternative is offered (examples: dozer.com, cipher.com)

What is the difference?

Thank you for helping
@Fred: Dear Fred, what are the sources of your input? Opinion? Work-experience? Experiences with domains? Knowledge of Guidelines? Guessing? Website (which link)? Class teaching? I heard about? Books? Films? Intuition? Thank you for augmenting your interesting primer

[Fred]

Some domains are not permitted for various reasons. Some are reserved for special use. But not available could just mean they are already in use as well, depends on the registrar's listing.